This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 2006-47036, filed on May 25, 2006 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to charge pumps, and more particularly, to a charge pump with reduced turn-off time, especially for use in a phase locked loop.
2. Background of the Invention
FIGS. 1A, 1B, and 1C show conventional charge pumps that are categorized as one of a drain switching type (FIG. 1A), a gate switching type (FIG. 1B), or a source switching type (FIG. 1C). Each category depends on the position of a respective set of switches (S1 and S2 in FIG. 1A, S3 and S4 in FIG. 1B, and S5 and S6 in FIG. 1C).
FIG. 2A illustrates a timing chart of control signals UP and DN for controlling such switches S1, S2, S3, S4, S5, and S6. FIG. 2B illustrates up (IUP) and down (IDN) current characteristics of the drain switching type charge pump of FIG. 1A. FIG. 2C illustrates the up (IUP) and down (IDN) current characteristics of the gate switching type charge pump of FIG. 1B. FIG. 2D illustrates the up (IUP) and down (IDN) current characteristics of the source switching type charge pump of FIG. 1C.
The source switching type charge pump of FIG. 1C typically exhibits the best up/down current matching, turn-on time, and turn-off time among the charge pumps of FIGS. 1A, 1B, and 1C. Accordingly, the source switching type charge pumps are widely used.
However, the source switching type charge pump in FIG. 1C does not have a discharge path for discharging residual charge remaining at the node between the switch S5 and a transistor M2 and at the node between the switch S6 and a transistor M1, during turn-off. Thus, such residual charge remains for a great amount of time such that the transistors M1 and M2 remain in a slight turn-on. When the source switching type charge pump is used in a phase locked loop, during a lock state, noise and spurious reference characteristics for an output terminal of the phase locked loop may be deteriorated from such transistors M1 and M2.
FIG. 3 illustrates output current characteristics of the source switching type charge pump of FIG. 1C during turn-on and turn-off. Referring to FIG. 3, turn-off time is very long in the source switching type charge pump.
FIG. 4 shows another conventional source switching type charge pump. FIG. 5 illustrates output current characteristics of the source switching type charge pump of FIG. 4 during turn-on and turn-off. Referring to FIGS. 4 and 5, the conventional source switching type charge pump of FIG. 4 includes a pull-up transistor P30 and a pull-down transistor N30 to discharge residual charge from intermediate nodes B1 and A1, respectively.
In the source switching type charge pump of FIG. 4 including the pull-up transistor P30 and the pull-down transistor N30, turn-off time is improved, but turn-on time is increased. In this case, mismatch between an up-current IUP—REF and a down-current IDN—REF is increased very high until transistors N11 and P11 are turned on sufficiently.
In addition, the conventional source switching type charge pump includes transistors assembled in cascode for increased output impedance. However, turning on the pull-up and pull-down transistors P30 and N30 may decrease the output impedance resulting in degradation of noise and spurious characteristics.